The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material having excellent coloring performance.
A conventional thermosensitive recording material comprises a support material, such as a sheet of paper or a film, and a thermosensitive coloring layer formed thereon, which comprises a coloring system consisting of a colorless or light-colored coloring dye such as a leuco dye and a color developer capable of inducing color formation in the coloring dye upon application of heat thereto such as a phenolic compound (for example, Bisphenol A) and an organic acidic material. In addition to the above coloring system, the thermosensitive coloring layer further comprises a binder agent, a filler, a thermal sensitivity improvement agent, a lubricant and other auxiliary agents. Examples of such thermosensitive recording material are disclosed, for example, in Japanese Patent Publication No. 43-4160, Japanese Patent Publication No. 45-14039, and Japanese Laid-Open Patent Application No. 48-27736.
In a thermosensitive recording material of this type, a colored image is obtained by a chemical reaction between a coloring agent and a color developer upon application of heat thereto, for instance, through a thermal head which is built in a thermal printer or a facsimile apparatus.
Such thermosensitive recording material has the advantages over other conventional recording materials that recording can be performed in a short time by use of a comparatively simple device without the need for complicated processes such as developing and fixing, almost no noise is generated and no environmental pollution problems occur during operation, and the cost is low. Because of these advantages, the thermosensitive recording material has wide-scale utilization, not only as the recording material for copying books and documents, but also as the recording material for use with computers, facsimile apparatus, telex, medical instruments, other apparatus for recording information, and measuring instruments.
In accordance with the recent demand for high speed recording with high recording density, not only the development of a high speed recording apparatus, but also the development of recording materials that can be used with such a high speed recording apparatus is desired.
A first method by which the above demand could be met is to decrease the melting point of an electron acceptor serving as color developer as much as possible for use in practice, for instance, to 80.degree. to 120.degree. C., taking into consideration the conditions for preserving the color developer, so that the co-melting initiation temperature with a leuco dye is decreased, thus it is tried to attain high speed recording. This method, however, will not be employed in practice, because it is extremely difficult to adjust the melting point of the phenolic compounds which are widely used as the color developer at present in the field of thermosensitive recording material, and because the cost of the phenolic compounds will become high if it is tried to do so.
A second method by which the above demand can be met to some extent is to add a thermofusible material to the thermosensitive coloring layer in such a manner that the thermofusible material is caused to work as a sensitizer or a melting point reducing agent for the thermosensitive coloring layer as described in Japanese Laid-Open Patent Applications Nos. 53-39139, 53-26139, 53-5636 and 53-11036. Examples of such a thermofusible material are various waxes, fatty acid amides, alkylated biphenyl, substituted biphenyl alkane, coumarine, coumarine derivatives, and diphenylamine. The density of the colored images, the coloring sensitivity and the degree of whiteness of the background obtained by the thermosensitive recording materials using such thermofusible materials in the thermosensitive coloring layer, however, are still poor.